New technologies can extend our scientific understanding. They can also mean we have to throw out earlier ideas.
Prior to the 1950s and the ‘radiocarbon revolution’, archaeology relied on forms of relative dating and the idea that older things are buried beneath younger things.
Professor Tom Higham is director of the Oxford Radiocarbon Accelerator Unit at Oxford University and Principal Investigator for the PalaeoChron Project. Tom grew up in New Zealand and completed his PhD at the Waikato Radiocarbon Dating Laboratory at Waikato University. His work has turned many ideas about early human evolution and migration on their head.
Tom’s work uses new methods for pre-treating samples before using radiocarbon dating, redating the fossilised bones that had provided the original evidence in previous research on early human evolution.
The movement of our earliest modern ancestors and their effects on other, now extinct, archaic humans living in Eurasia – such as Neanderthals and, of course, now the Denisovans – is one of the most important questions in human evolution studies. In order to understand the period, we need a sturdy chronology underpinning the archaeology and molecular biology.Prof. Tom Higham
The Palaeolithic period, also known as the Old Stone Age, is the prehistoric period of human history distinguished by the use of stone tools, from about 2.5 million years ago to 10,000 years ago. It is also the period of history when the early hominins of the genus Homo habilis– such as Homo habilis – gradually evolved to anatomically modern humans – Homo sapiens.
The early part of the period, the Lower Palaeolithic, had hominins using basic stone tools. In the Middle Palaeolithic, fire was in use, and there is evidence of the cooking of food. Towards the end of this period, the Upper Palaeolithic, humans began to produce art such as cave paintings, rock art and jewellery and began to engage in ritual behaviour such as burial of the dead.
The Palaeolithic period is also when hominins began to migrate from Africa into Europe, Asia and eventually the rest of the world. This migration to different environments and climates would have in part driven evolutionary adaptation as well as the extinction of different hominins.
Dating Palaeolithic artefacts is important in order to piece together this part of the story of human evolution and migration. Radiocarbon dating can only date as far back as the Upper Palaeolithic – 50,000 years ago. However, dating Upper Palaeolithic artefacts is challenging because radiocarbon techniques depend on the presence of C-14, and when you get down to 50,000 years ago, you’ve got 0.1%, meaning the tiniest amounts of contamination can significantly impact calculations.
The problem of contamination
Any sample for carbon dating can be contaminated by artificial or natural sources of carbon.
Any carbon-containing material that affects the C-14 content of a sample is a contaminant that can cause inaccurate dates.
Archaeological artefacts like bone fossils are usually found embedded in or buried with other materials that may have affected their radiocarbon content. For example, the roots of plants can penetrate bones, introducing a new carbon source. This ‘natural’ contamination can make the samples seem younger than their true age. Alternatively, contamination from limestone (which is made from organic materials) can make a sample appear much older than it is.
Artificial contaminants are introduced by people, for example, during excavation or field conservation or when packaging the samples. Labelling bone samples with animal glue is an example of artificial contamination.
In the drive for better accuracy, many pre-treatments have been devised to clean materials of any contaminants prior to their dating.
The need to more accurately date bone samples led to pre-treatments such as the removal of the organic material from the bone – collagen protein. Prior to this, the entire bone was ground up and used – even though the most reliable carbon was in the 30% that was collagen.
In the 1980s, a technique called ultrafiltration was developed to purify collagen.
An ultrafilter is a molecular sieve that separates high and low molecular weight (MW) fractions. Low MW components can include degraded amino acids and peptides and soil-derived contaminants, all of which are discarded after separation.
Roger Jacobi, a Palaeolithic archaeologist colleague of Tom, approached him with questions about some unexpected radiocarbon dates from several sites in the UK. Tom and his team started to reinvestigate the dates using their improved chemical pre-treatment methods. They noticed some significant differences between the previous dates and the newly produced ones. Sometimes the differences were thousands of years. Tom realised they would need to recalculate previous dates with more robust approaches, so they applied for funding and the PalaeoChron Project was born.
The PalaeoChron Project aims to refine and improve the chronology of Palaeolithic sites from Western Europe to Siberia.
We want to create this huge map that will allow us to look at the movement of people, the movement of objects, the development of new ideas – the big archaeological questions really.Prof. Tom Higham
Tom’s team extracted collagen and used ultrafiltration to pre-treat over 400 Upper Palaeolithic samples from across 40 archaeological sites in Europe.
In some cases, they tested bones that had been previously dated, and in other cases, they dated bones found above these previously dated bones.
The calculated ages were very different from the original dates. In some cases, more than 70% of dates at some of the sites they were looking at were “plain wrong”.
Many of the Neanderthal bones were far older than originally dated. For example, an almost complete Neanderthal infant skeleton found at Mezmaiskaya in the Russian Caucasus was dated at just over 29,000 years BP. When Tom’s team dated Neanderthal bones from the same collection, found above the infant skeleton, they were expecting a younger date but they got a much older date of 40,000 years BP. This dating was repeated four times, and each time the date was the later one of 40,000 years ago.
You have to know the dates.Prof. Tom Higham
Rewriting our evolutionary history
The new dates indicate that Neanderthals died out in Europe 10,000 years earlier than previously thought – between 41,000 and 39,000 years ago. These dates coincide with the start of a very cold period in Europe.
The dates also show an overlap in Neanderthal and modern human populations of around 5,400 years. This is significant, as previous dating calculations indicated that the populations had only overlapped by about 500 years. This means that ideas around the rapid extinction of Neanderthals caused solely by the arrival of modern humans are no longer likely. The overlap between the two populations also means that Neanderthals and anatomically modern humans could have shared cultural ideas. This is further supported by genetic evidence of inbreeding between modern humans and Neanderthals.
The new dates also suggest that modern humans arrived in Europe several thousand years earlier than previously thought, possibly as early as 45,000 years ago, meaning that Neanderthals were already in decline.
The PalaeoChron Project is built around a suite of methods for refining radiocarbon dating calculations. Optically stimulated luminescence (OSL) is used to measure the age of sediments from excavation sites. OSL can measure back 200,000 years and is useful where the 50,000-year limit of radiocarbon dating is reached.
Another method the PalaeoChron Project has been working with uses collagen peptides – short-chain amino acids found in bone – to identify the mammal origins. Fragments of bone from archaeological digs are often too small to be identified by their morphology and can have been further degraded by having passed through the digestive tract of another animal. The method, called zooarchaeology by mass spectrometry (ZooMS), uses chromatography to separate out and identify the collagen peptides using high-performance liquid chromatography (HPLC).
This method allows researchers to sort hominin bones from those of other animals. The method is also minimally invasive in that only a tiny fraction of the original sample is required, meaning that remaining fragments can be used in different analytical procedures, such as for radiocarbon dating and genetic material for genome sequencing.
The radiocarbon revolution
Radiocarbon dating developed in the 1950s and had a huge impact on archaeology. It was referred to as the ‘radiocarbon revolution’.
Carbon dating relies on the presence of C-14, a radioactive form of carbon. Organisms stop absorbing C-14 when they die, and so the ratio of C-14 to C-12 can be calculated. This is because C-14 radioactively decays to C-12. The half-life of C-14 is 5,730 ± 40 years, meaning that it takes 5,730 years for half the C-14 to decay. The ratio of C-14 to C-12 is used to calculate the age of the organism.
In the late 1970s, a refined technique using smaller samples was developed – accelerator mass spectrometry (AMS).
Athol and a colleague also played an important role in recognising the increase in radiocarbon in the atmosphere as a result of above-ground nuclear weapons testing in the 1950s. This allowed for changes in calibration to ensure better accuracy.
Nature of science
The research explored in this article highlights the tentative nature of science and that scientific ideas need to be re-examined when new evidence comes to light. It also shows the importance of replication and of multiple types of evidence. For example, Tom’s team compared radiocarbon dating calculations with assumptions based on the layers in which samples were found (higher layers should be younger than lower layers).
The Science Learning Hub would like to acknowledge the Allan Wilson Centre for Molecular Ecology and Evolution, which sponsored and recorded Professor Tom Higham’s lecture When Neanderthals and Modern Humans (and Denisovans) Met: Human Evolution from 60,000–30,000 Years Ago in September 2015.